Download 1 LAB # 9 - THE INVERTEBRATE

LAB # 9 - THE INVERTEBRATE DEUTERSTOMES
1. Overview
We will consider 2 phyla of Deuterostomes in this lab: the Echinoderms (Phylum
Echinodermata) and the tunicates (Phylum Urochordata). These 2 groups represent the first
major shift from the protostome line of evolution (the annelids, molluscs and arthropods), in a
line that also includes the chordates and thus, vertebrates. Be sure to familiarize yourself with
your notes from the beginning of term that describe the differences, and significance, of this
major transition in the history of animal life.
•
•
•
•
2. Essential features of the lab
dissect Pisaster and Cuccumaria
understand the comparative functional morphology of the 5 Echinoderm classes and be able
to describe the general characteristics of the tunicates
understand the design and significance of the Echinoderm water vascular system
slide material, focusing on larval life-history stages
4. Special Features
a. General biology and functional anatomy of selected Deuterstomes
Phylum Echinodermata
Members of this phylum are all marine and free-living. There are no parasitic or colonial species.
Their invasion of land and freshwater has been prevented by their cutaneous gas exchange
methods and their lack of distinct osmo-regulatory organs. They comprise the major invertebrate
group of deuterostomes. The larvae are bilaterally symmetrical, while adults exhibit
pentaramous (five-fold) symmetry. Thus, the body is arranged into 5 parts that radiate from a
central axis. All echinoderms have a calcareous endoskeleton with a dermal layer of ossicles and
external spines on the body surface. The other diagnostic feature is the presence of a unique
water vascular system that bears the sucker-like tube feet. The water vascular system is a
manifestation of the well-developed coelom. The design of the water vascular system functions
in locomotion, feeding, respiration, and excretion. In deep sea habitats, these invertebrates may
comprise more than 90% of the benthic biomass.
Most echinoderms are dioecous with relatively simple reproductive systems, intimately
associated with the coelom. Life history strategies vary from free spawning with external
fertilization and indirect development (i.e. a planktonic stage) to various forms of brooding, with
direct development. Because echinoderm larvae are easy to raise in the lab, their embryology is
quite well known. Great interest has been placed on the precise mechanisms by which the
bilaterally symmetrical larvae metamorphose into radially symmetrical adults! Larval types vary
within and among classes but one of the most characteristic is the motile bipennaria.
Class Stelleroidea
Sub-Class Asteroidea
1
This class includes the familiar sea stars. Most are predatory carnivores that specialize on
bivalve prey. Some, such as the purple Sea Star from the West Coast act as so-called Keystone
predators within marine ecosystems.
Asteroideans are flattened, star-shaped echinoderms with an madreporite (the entry into the
water vascular system) and cleansing, pincer-like pedicellariae. The hollow arms are not
strongly demarcated from the central disc and the arms have hepatic cecae that are important in
digestion. Examine the slides of pedicellariae.
Sub-Class Ophiuroidea
These are the brittle stars. These echinoderms fragment when disturbed, hence the name. The
solid arms are strongly demarcated from the central disc, in which the digestive system is found.
Note the protective calcareous plates (or shields) on the aboral surface. A unique feature is the
10 invaginations on the oral surface known as bursae. Seawater constantly circulates through
these bursae to provide a mechanism of gas exchange and waste elimination. In many species,
these bursae serve as brood chambers within which embryos develop. These are the most mobile
members of the phylum. Ossicles on the arms form a continuous articulated armour. Most brittle
stars feed on detritus using a variety of mucous mechanisms to trap food.
Class Crinoidea
This group includes the sea lilies and feather stars, a group that is mostly restricted to deepwater habitats. The arms are branched and the oral surface bears the mouth and anus, which both
face upwards. Sea lilies are suspension feeders, remaining attached to the substrate by a flexible
stalk composed of calcareous discs. Note that the feeding and reproduction portion of the animal
is situated at the top of the stalk that forms a cup-like structure called the calyx. Note that
several arms project from the calyx, each of which possesses pinnules and tube feet. These are
used to collect food particles from the surrounding water. At the top of the stalk is the calyx.
Feather stars resemble the sea lilies from the calyx upward. However, instead of a long stalk, a
series of jointed, flexible appendages called cirri are located at the base of the animal. Feather
stars use a coordinated series of arm movements for swimming locomotion. The stalked
crinoids, on the other hand, are unable to move to avoid predators.
Class Echinoidea
This group includes the sand dollars and sea urchins. All have discoid body shapes and no
arms. All possess a unique feeding structure known as Aristotle’s lantern. Sand dollars have
flattened tests and their tube feet and spines are greatly reduced. They have an Aristotle’s
lantern, but it is adapted for feeding on smaller particles of food. It receives these particles from
ciliated grooves that deliver food that has fallen onto the surface of the sand dollar as it moves
through the sand.
On a sea urchin, note the 5 moveable jaws that make up the powerful Aristotle’s lantern. It uses
this structure to graze algae. Study this complex apparatus with your dissecting microscope.
Note other characteristic external features of preserved sea urchins and note the dissected
example of an Aristotle’s lantern.
2
Class Holothuroidea
These are the familiar sea cucumbers. All have a cylindrical body and lack arms. They also
have modified tube feet which form oral tentacles. They have an endoskeleton with tiny ossicles
embedded in a leathery body wall.
b. Dissections (see Chapter 20 in Pechenik for appropriate diagrams)
Pisaster
The oral surface normally faces the substrate and contains a central mouth, which is surrounded
by a peristomal membrane, and is guarded by prominent oral spines. Extending into the oral
surfaces of the five rays from the region of the mouth are five ambulacral grooves. These also
are guarded by ambulacral spines, and contain a number of rows of fleshy tube feed (podia).
Examine the podia under a dissecting microscope, and note the sucker-like distal tip. The podia
are the external portions of the water vascular system of the starfish.
The aboral surface is covered by an epidermis that is broken by calcareous spines that project
outwards from the dermal ossicles. These make up the endoskeleton of the starfish. Locate the
circular madreporite that is located just off-centre from the aboral axis. This is the opening of
the water vascular system that is at the opposite end of the system from the tube feet. The two
arms nearest the madreporite are the bivium; the remaining three are the trivium. There is a
vestigal anus on the aboral surface but it is too small to be seen.
Examine a scraping of epidermis from the aboral surface in a drop of water using a compound
scope. Try to find the opaque, pincer-like structures called pedicillariae. Some are 2-piece
pincers, others are 3-piece with a basal part on which the two pincers articulate. Compare these
pedicillariae with ones from prepared slides. What are the function of the pedicillariae?
Also present on the aboral surface are thin bladder-like structures which extend up into the
surrounding water; these are the dermal branchiae or skin gills. Usually they are retracted in
preserved starfish, and may not be observable at all. These respiratory exchange surfaces are
filled with coelomic fluid, and can be retracted if anything disturbs the surface of the starfish.
Examine the tips of the arms for the eyespot (these may also be difficult to see on preserved
specimens).
Using heavy scissors, cut through the body wall along the sides of the arms. The body wall is
particularly thick where 2 arms adjoin the central disk. Extend your cut along the aboral surface
of the central disc in such a way that when the aboral body wall is removed, the surface of the
bivium, the madreporite, and a bit of body wall around the madreporite, are left intact.
Carefully lift up the aboral body wall. Note that the organs exist in a spacious coelom. The inner
surface of the body wall is covered by a coelomic epithelium which is a peritoneum. The
antler-shaped rectal cecum may be observable near the centre of the aboral axis; it is about 5mm
long, branched, somewhat glossy, and reddish in colour. It attaches at the junction of the
rudimentary intestine (hard to see) and the aboral portion of the stomach, the delicate, almost
star-shaped pyloric stomach. This is connected via pores to five sets of paired pyloric cecae,
located aborally in the arms. Oral to the pyloric stomach is the more muscular, 5-lobed cardiac
stomach. This is the part of the stomach that is everted during feeding. On the ambulacral ridge
3
(the roof of each ambulacral groove as observed from the coelomic side) in each arm originates a
delicate pair of cardiac retractor muscles that insert in the lobes of the cardiac stomach. They
can be observed most easily by gently lifting a lobe of the cardiac stomach, which in turn lifts the
muscles away from the roof of the ambulacral ridge, thus making them apparent.
Identify the paired gonads occupying the floor of the body cavity in the proximal region of each
arm. Although the starfish are dioecous it is not possible to sex them macroscopically.
Beginning with the madreporite, trace the parts of the water vascular system. From the
madreporite, a whitish slender tube, the stone canal, extends orally to connect with a ring canal
which is embedded in the bony ossicles which surround the mouth. Projecting from the inner
surface of the ring are 9 Tiedemann bodies. It will be necessary to remove (carefully!) the
cardiac and pyloric stomachs to trace these parts. From the radial canals extend large numbers of
lateral canals (visible only in sectioned material) which connect with the ampullae (bladders)
and tube feet. The rounded ampullae can be seen on the coelomic side walls of the ambulacral
ridge. In some cases you can cause the ampullae to swell by pressing on their associated tube
feet, and vice versa.
With a sharp scalpel, make a clean cut through one of the bivial arms, close enough to the central
disc to include a section of gonad. Compare the structures observed here with those seen in
prepared microscope slides.
Cucumaria
Dissect a preserved sea cucumber by referring to the appended figures. Think of these unusual
echinoderms as typical forms, but ones that are elongated on an oral/aboral axis. The oral end
contains the mouth and mucus-covered tentacles (just modified tube feet) that can usually be
retracted into the body. The spineless body wall of most sea cucumbers i leathery and contain an
embedded endoskeleton of microscopic, calcareous ossicles (try to find these under a squashed
preparation under the scope). Tube feet are arranged in 5 ambulacral rows (2 on the dorsal
surface; 3 on the ventral). Motion is by muscular contraction of their body, aided by tube feet.
Make your incision from posterior to anterior and then pin the sides back. Refer to the attached
figures for help in understanding the functional morphology associated with feeding, respiration,
reproduction, musculature and water vascular system. Note the circular and longitudinal muscle
layers. These are considered edible delicacies! Note that the respiratory tree connects to the
cloaca which pumps water into the tree for gas exchange. Many sea cucumbers respond to a
variety of physical and chemical stimuli by eviscerating (this is why they are also known as
Holo-throw-up-ians). In many species the entire digestive system, along with the respiratory
trees and gonads may be expelled in response to predators. All lost parts are then regenerated,
reflecting the phenomenal regenerative capabilities of most echinoderms. Interestingly, one
species of parasitic, shelless gastropod lives within the coelom of some sea cucumbers on the
west coast. It uses periods of host evisceration to release its larvae !
c. Echinoderm diversity
Pisaster - This is the most conspicuous sea star in rocky intertidal areas on the West Coast. It’s
main prey are mussels, but they will also take large gastropods, limpets and chitons. They show
4
remarkable colour polymorphisms, ranging from bright orange to deep purple. Recall the size of
the anterior and posterior muscles of Mytilus that we dissected earlier. Remarkably, Pisaster uses
a combination of the holding and pulling action of hundreds of tube feet, plus the ability to evert
its stomach between the valves, to open its prey.
Dermasterias - This is the leather star, also common on the West Coast. It has short, thick arms
and the texture of its upper surface is soft and slippery. Its tube feet are usually adapted as
suckers and they lack pedicellaria.
Solaster - This is the sun star, a large and distinctive species on the West Coast. One species of
Solaster is one of the few ‘specialist’ echinoderms, feeding exclusively on sea cucumbers.
Another species feeds exclusively on a different species of Solaster !
Pycnopodia - These species can reach diameters of up to 1 m. It is one of the most mobile of all
echinoderms and is an important marine predator. It is a feeding generalist, but tends to
concentrate on bivalves. When the tube feet of this species touch a sea cucumber, it writhes
vigorously to escape.
Ophioderma - this species is an active predator, capturing benthic prey by curling an arm into a
loop around the prey, then pulling it to the mouth. Spines located on the arm function to
puncture the prey.
Gorgonocaphalus - These are the basket stars. They are nocturnal, predatory suspension feeders
that assume a feeding position with their branched arms held like fans in the current. When small
crustaceans or polychaetes contact an arm, the appendage curls to capture the prey. Prey is then
transferred to the mouth.
Strongylocentrotus - members of this genus include the green and purple sea urchins. Both are
very abundant on the west coast in rocky areas as well as subtidally on various substrates. Purple
sea urchins are especially common on exposed headlands where they sometimes imbed
themselves directly into rock. They do this by continual action of their moveable spines and the
continual release of calcareous material from the tissue at the end of the spines. Both species are
major prey of sea otters. When populations of this predator were reduced by trappers,
populations of sea urchins exploded. One result was the destruction of kelp beds (their main
prey), one of the primary sources of invertebrate and vertebrate biodiversity on the west coast.
Dendraster - This is the common species of sand dollar found on beaches on the West Coast. It is
generally a suspension feeder, although it will also capture small invertebrates with its
pedicillaria.
Parastichopus - this is the species we often have in the marine tank. It can reach lengths of up to
40 cm on the west coast. It is a rather atypical sea cucumber, in that it lacks the typical tentacles
at the anterior end and has an abundance of pointed, fleshy warts. The tube-feet are crowded
together and only 3 of the 5 rows are usually distinct. The specialized tube feet around the
mouth have the tips elaborated into small discs. As detritus is collected on the tube feet, they are
inserted into the mouth and ‘licked’ clean.
5
Cucumaria - this is the species we will dissect. It grows to a length of about 20 cm and is
common on the west coast, typically living in crevices or rocks. Often, only the bright orange
crown of tentacles is visible underwater. The oral tentacles are branched, functioning to trap
suspended particles. Once a tentacles is saturated with food, it is brought to the mouth and
‘cleaned off’.
Psolus - the scaly sea cucumber is also found in the intertidal on the West Coast. It is actually a
beautiful species, especially when its bright red tentacles are extended. Many people do not
recognize it, on sight, as a sea cucumber, but a chiton (due to the distinctive, overlapping
calcareous plates. Think of it as a short cucumber, sliced lengthways ! The side with the tube
feet is almost perfectly flat. It is essentially sessile, clinging tightly to rocks. The mouth is
situated on the side with the plates, some distance from the anterior end.
Phylum Chordata
Subphylum Urochordata - Class Ascidiacea
The Deuterstome line of evolution contains only a few phyla, with the Vertebrates and the
Echinoderms dominating in every way. Among the Chordates, by far the dominant group is the
Vertebrates (95% of species); the other two are the Chaetagnatha and the Hemichordata. In this
section, we consider a well-known subphylum of chordates, represented by the tunicates. They
too, are somewhat obscure. However, what they lack in terms of species numbers and ecological
importance, they make up for in their phylogenetic position as a link between invertebrates and
vertebrates.
The chordates are characterized by pharyngeal gill slits, a hollow nerve cord, and a dorsal
supporting rod called a notochord at some point in their life cycle. The larval tunicates (but not
the adults) have a notochord and a dorsal nerve cord and the adults have enlarged pharyngeal
region that has one or many gill slits. Tunicates may be benthic or pelagic.
The Ascidiaceans, known as sea squirts or tunicates are common on the west coast and in all
warm oceans and seas (where they are often mistaken for sponges). They can be colonial or
solitary and are often highly coloured. They are all hermaphoridites that produce a motile, tailed
larvae that swims for a period before metamorphosing into an adult. The example we will use is
Molgula. It is typically a pale green colour; fixative turns them into boring ‘olives’. Notice the
large, incurrent siphon at the distal end. The excurrent siphon is lateral to it. The outermost layer
(or tunic) is secreted by the epidermis.
6